iPhone 7 Apple logo stuck/ loop after housing change?

As simple as it seems, a lot can go wrong with a housing swap.

You say you checked all the flexes, by that I assume you mean you inspected them closely to insure there are no tears, even tiny ones. Did you also look at the connectors themselves, both on the flexes and logic board?

It's not quite clear from your question but did you try to use the logic board in another device? The challenge here is that you have so many variables. You need to find a way to start eliminating a few of them. For example, if you have a known-good, working device, start moving over one flex at a time until the device no longer works. You also need to determine if the issue is with the logic board (for example, you may have damaged something when prying off connectors) or with a flex.

The horizontal white line is worrisome.